This application relates generally to methods and devices for displaying images and more particularly to fiber-optic display devices.
Optical fiber display devices are commonly used to display large images or in the type of situation remote viewing is required. Fiber-optic displays can receive an image from a source device, such as an image projector, and subsequently redisplay the image on an output screen by means of optical fibers.
Optical fibers, also known as light guides, are long, thin threads of fused silica or other transparent material that transmit light projected onto one end of the fiber to the other end of the fiber. Optical fibers are typically flexible and have a circular cross-section and a cylindrical surface connecting the two ends. In a fiber-optic display, a multitude of optical fibers is collected into a bundle. The bundle has an input surface where one end of each optical fiber is held in place, typically by a plastic or composite substrate, so that they form a generally planar surface. Likewise, the bundle has an output surface where the other end of each optical fiber is held in place to form another surface. Each fiber has a specific position relative to the others at both surfaces so that an image projected on the input surface is transmitted to and redisplayed at the output surface. Typically, the fibers are held close together at the input surface and are spaced father apart on the output surface, which has the effect of enlarging the displayed image relative to the source image.
A number of factors may degrade the quality of an image displayed by a typical fiber-optic display. For instance, ambient light may reflect from the substrate that holds the optical fibers at the output surface. The reflected light, referred to as glare, degrades the quality of the displayed image by reducing the contrast of the displayed image and otherwise interfering with the viewing of the displayed image.
The problem of glare may be exacerbated by the use of diffusion layers that increase the viewing angle of fiber-optic displays. Optical fibers emit light in a very tight beam, which results in a limited viewing angle that is restricted to the viewing angle of the raw fiber-optic element. Viewers outside of the limited viewing angle of the raw fiber-optic element only see the output surface of the bundle and do not see the displayed image. A typical solution to the viewing angle problem is to disperse the emitted light by the addition of a uniform diffusion layer, alternately referred to a dispersion layer, on the output surface. This layer typically consists of a thin sheet of material, referred to as a diffusion sheet, that uniformly refracts or otherwise disperses the light emitted from the output surface of the bundle and thus widens the viewing angle. However, the addition of the diffusion layer may increase the detrimental effect of glare on the image quality by diffusing the reflected glare as well as the light emitted from the optical fibers.
Additionally, the inclusion of a diffusion layer may introduce additional problems in large fiber-optic displays. In large applications, the diffusion layer may be made of multiple, smaller diffusion sheets because a single, large diffusion sheet can be difficult to apply uniformly. Any non-uniformities in the diffusion layer, either due to entrapped air between the diffusion sheet and the output surface, or creases in the diffusion sheet, adversely affect the viewing quality of the image by causing non-uniform dispersion of light emitted by the optical fibers. However, when multiple diffusion sheets are used, the joints between sheets can be another source of non-uniformities in the diffusion layer. The non-uniformities can be due to the overlapping of the diffusion sheets at the joints or enlarged gaps between diffusion sheets, which cause light emitted from some optical fibers to be dispersed more or less than the light from other optical fibers. In addition, gaps between the diffusion sheets may be more evident because of the direct and undispersed glare from the exposed substrate.
Another problem with optical fiber displays is due to the loss of a portion of the light projected upon an end of an optical fiber from the surface of the optical fiber during the transmission of the projected light to the other end. Transmission loss is a function of the length and optical characteristics of the optical fiber and reduces the contrast and brightness of the displayed image. In addition, the transmission losses from one fiber can subsequently penetrate an adjacent fiber in the bundle, a phenomenon referred to as crosstalk, which also degrades the final quality of the displayed image.
The present invention overcomes the disadvantages and limitations of the prior art by providing various embodiments that increase the quality of images displayed by fiber-optic displays. In one embodiment, diffusion sheets have been developed that contain one or more light-passing and light-absorbing regions. These regions are created by selectively covering regions on a surface of the diffusion sheets with a light-absorbing material, such as black paint. Light can pass through the uncovered regions of the diffusion sheets but not through the covered regions. The light-absorbing material can be located on either surface of the sheet, or may, in some cases, even be incorporated within the diffusion sheet. In the preferred embodiment, the covered regions are located on the surface of the diffusion sheets that is opposite the output surface of the substrate. The covered regions prevent glare from the substrate and diffusion layer by absorbing ambient light that would otherwise enter the diffusion layer and be reflected from the substrate. The uncovered regions of the sheets are located between the covered regions and are aligned with the optical fiber ends so that light emitted from the fibers is not covered or absorbed by the light-absorbing material and is transmitted through the diffusion sheet.
For applications requiring multiple sheets, the sheets are made in a diamond- or triangle-shaped configuration which allows their application to the substrate surface with an increased margin for error in positioning of the sheets when compared to rectangular sheets. It has also been determined than the diamond and triangle shapes are less likely to entrap air between the sheets and the substrate during application than a rectangular shape of the same area. Furthermore, because there are greater tolerances in placing the sheets on the substrate, the sheets may be applied to the substrate without overlap and with a small gap between adjacent sheets. This greater degree of tolerance also prevents fiber optic elements from being partially covered. The surface of the substrate between the gaps can also be treated to reduce the amount of light reflected from the exposed substrate.
The performance of optical fiber bundles has been improved by coating the surface of the individual fibers except for the ends with a reflective material, preferably silver colored, to prevent escape of light from the fibers. In an alternative embodiment, a light-absorbing material, preferably a black-colored adhesive, is applied to the surface of the fibers, except at the ends, to prevent crosstalk interference.
Another aspect of the invention is the development of a substrate that prevents bending losses by holding the fibers parallel within the substrate for a calculated minimum distance from the ends of the fibers. In the preferred embodiment the fibers are held parallel for a distance of at least, approximately, 100 times that of the diameter of the fibers.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.